1. Field of the Invention
This invention relates to an information signal reproducing apparatus, and more particularly to an apparatus for reproducing an information signal which has been recorded along with one of four pilot signals, having different frequencies from one another, for every successive recording track in a predetermined order.
2. Description of the Prior Art
In the following specification, the information signal reproducing apparatus will be described using the video signal recording and reproducing apparatus. It will use a magnetic tape as the recording medium and a rotary head (hereinafter referred to as "VTR").
In conventional VTRs, for precisely tracing the same recording track during reproduction as when recorded on the recording medium, there are two methods which are either by recording control signals in the form of a pulse train, or the so-called CTL on the vicinity of the magnetic tape, or by superimposing a reference signal of a certain frequency on the video signal during recording.
The present invention involves VTRs which make use of four pilot signals of different frequencies from one another. During recording, these pilot signals are superimposed on the video signal successively, in a prescribed order, at a rate of one for every recording track.
In VTRs employing such a recording system, when switched to the reproducing mode, a reference signal, of the same frequency as that of the pilot signal, which was recorded in the principally traced recording track (hereinafter called a "reproducing track"), is multiplied by a signal from the reproducing head to obtain crosstalk components of the pilot signals for the respective adjacent tracks on either side of the reproducing track. By detecting a difference in levels between the crosstalk components, adjustment of the head position to the reproducing track is controlled.
FIG. 1 in a block form illustrates an example of the reproducing system of the prior art VTR employing such a method. The reproduced signals from heads 1 and 2 pass through a head selector circuit 11 to an amplifier 12. After having been amplified, the signal is converted to a video signal equivalent to the original one, or the video signal which was recorded in passing through a signal processor 13, and the reproduced video signal appears at an output terminal 3. The output of amplifier 12 is also applied to a low pass filter (LPF) 14 where only the above four pilot signals are allowed to pass therethrough to a multiplier 15. A signal appearing at an inlet 16 was formed in synchronization with the operation of a rotary drum with the heads 1 and 2 fixedly mounted thereon and has alternative high and low levels for a period necessary to trace one track, for example, at a frequency of 30 Hz, (hereinafter called 30PG). The 30PG is also used to control the operation of the head selector circuit 11. In synchronization with the 30PG, the circuit 17 produces the four different reference signals at a time which has the same frequency as that of the pilot signal recorded in the track which is being traced by either head 1 or head 2. The reference signal obtained by this circuit 17 is applied to the multiplier 15. That is, the 4-frequency reference signal forming circuit 17 produces four reference signals of different frequencies successively in the same frequency order as the pilot signals were recorded in the respective successive tracks.
In the multiplier 15, the reproduced pilot signals from LPF 14 are multiplied by the reference signals from the 4-frequency reference signal forming circuit 17. The reproduced pilot signals each include crosstalk components ascribable to the adjacent tracks on either side of the reproducing track. Here the four frequencies are assumed to have been recorded in an order of f1.fwdarw.f2.fwdarw.f3.fwdarw.f4.fwdarw.f1.fwdarw.f2 and so on, taking respective values, for example, f1=102 KHz, f2=119 KHz, f3=165 KHz and f4=148 KHz. Then, as shown in Table 1, obtained from the output of LPF 14 are not only the pilot signal which was superimposed on the reproducing track during recording, but also, another pilot signal which had been recorded just before the first pilot signal was recorded on the reproducing track. In other words, the second pilot signal was superimposed on the track just behind the reproducing track (hereinafter called the front track, as viewed in the running direction of the tape) and still another pilot signal was recorded just after the first pilot signal had been recorded, or was superimposed on the track just before the reproducing track (hereinafter called the rear track).
TABLE 1 ______________________________________ Frequency in KHz ______________________________________ Pilot Signal of Reproducing 102 119 165 148 Track Crosstalk Component from 148 102 119 165 Front Track Crosstalk Component from 119 165 148 102 Rear Track Reference Signal 102 119 165 148 Spectrum Arising from Crosstalk 46 17 46 17 Component from Front Track Spectrum Arising from Crosstalk 17 46 17 46 Component from Rear Track ______________________________________
On the other hand, the reference signal produced by the 4-frequency reference signal forming circuit 17 is one which has the same frequency as that of the pilot signal of the reproducing track.
For example, now assuming that the frequency of the pilot signal of the reproducing track is 119 KHz, then the crosstalk component from the front track has a frequency of 102 KHz and the crosstalk component from the rear track has another frequency of 165 KHz. As that signal and those components are multiplied by the reference signal of 119 KHz, the output of the multiplier 15 includes 238 KHz, 221 KHz, 284 KHz, 17 KHz and 46 KHz, where the components of 17 KHz and 46 KHz are spectra produced for the reason that the crosstalk components from the front and rear tracks are respectively present, and their levels represent the amount of crosstalk. The 17 KHz and 46 KHz components are taken out by band pass filters (BPF) 18 and 19 respectively. Responsive to the outputs of BPFs 18 and 19, detector circuits 20 and 21 produce outputs the difference of which represents a positional error of the head to the reproducing track. Then the outputs of detectors 20 and 21 are applied to a difference amplifier 22 controlling the operation of a capstan motor 26 until the level difference obtained by the difference amplifier 22 reaches a minimum. As is obvious from Table 1, that the spectra 17 KHz and 46 KHz, arising from the crosstalk components from the front and rear tracks, alternate each time the head moves one track. For this reason, according to the prior art, it is necessary to provide an inversion amplifier 23 and still further an additional switch 24, which changes its switched position for each track. The operation of the switch 24 is controlled by the 30 PG. Then, the signal from the switch 24 is supplied to a control circuit 25 for the capstan motor 26, thereby making it possible for the head 1 or 2 to precisely trace the same track during reproduction as during recording.
FIG. 2 illustrates another example of the prior art reproducing system which is different from the example of FIG. 1 in that instead of the inversion amplifier 23 and the switch 24, use is made of a pair of switches 27 and 28, operation of which is controlled by the 30PG. Reference numeral 29 identifies a difference amplifier. Even in this system, the frequencies arising from the crosstalks of the front and rear tracks alternate for every track. Therefore, the polarity inverts each time the head shifts one track.
Since such prior art VTRs are necessarily provided with means for fulfilling the requirement of performing the above-described inversion of polarity, by an inversion amplifier and a switch in combination, or otherwise by two switches excluding the inversion amplifier, the complexity of the circuitry is increased, and the temperature response characteristic of the inversion amplifier and the noise of the switch or switches directly affect the signal representing the detected positional error of the head to the reproducing track. Even if the actual positional error is the same, it is unavoidable that the detected positional error differs from track to track, that is, as the switches 24, 27 and 28 assume their opposite switched positions.
An object of the present invention is to provide an information signal reproducing apparatus which overcomes the above drawbacks of the prior art.
Another object is to provide an information signal reproducing apparatus in which the electrical circuitry is constructed in a simpler form.
Still another object is to provide an information signal reproducing apparatus in which the signal representing the detected positional error can be stabilized.
A further object is to provide an information signal reproducing apparatus amenable to the effective use of electrical circuitry.